Use strong types in runtime for parachain heads and validation code (#964)

* use stronger types for HeadData and ValidationCode in runtime

* fix weird debug compile error

* fix runtime build

* update invocations invalidation.rs

* fix tests

polkadot/primitives/src/parachain.rs

@@ -214,7 +214,7 @@ pub struct CandidateCommitments {
 	/// The root of a block's erasure encoding Merkle tree.
 	pub erasure_root: Hash,
 	/// New validation code.
-	pub new_validation_code: Option<Vec<u8>>,
+	pub new_validation_code: Option<ValidationCode>,
 }
 
 /// Get a collator signature payload on a relay-parent, block-data combo.
@@ -672,7 +672,7 @@ sp_api::decl_runtime_apis! {
 		/// Get the local validation data for a particular parachain.
 		fn local_validation_data(id: Id) -> Option<LocalValidationData>;
 		/// Get the given parachain's head code blob.
-		fn parachain_code(id: Id) -> Option<Vec<u8>>;
+		fn parachain_code(id: Id) -> Option<ValidationCode>;
 		/// Extract the abridged head that was set in the extrinsics.
 		fn get_heads(extrinsics: Vec<<Block as BlockT>::Extrinsic>)
 			-> Option<Vec<AbridgedCandidateReceipt>>;